All rotating and reciprocating machines vibrate, in general, as a result of imbalances inherent in the machines, and although careful designing and balancing of the equipment can limit vibration, it is common practice to mount such equipment on vibration isolators to isolate the floor or other supports on which they are mounted from the equipment. It is also common to mount equipment that is sensitive to vibration on vibration isolators to isolate the equipment from vibrations of the floor or other structure. The technology of vibration isolators is highly developed, and the solutions to particular problems in specific installations is ordinarily a matter of engineering skill, coupled with judgments and compromises based on experience.
The frequency of vibration of the equipment varies with operating speed and must, of course, be considered as an important part of the design of the isolator. Normally, the natural frequency of the isolator is established well below the normal operating frequency of the vibratory equipment to ensure against resonant operation and excessive amplitude of vibration. However, in starting up or shutting down the equipment, there are unavoidable transient vibrations that are in resonance with the natural frequency of the isolators on which the equipment is mounted. Under transient conditions, if the weight of the isolated equipment is not very large as compared with its residual unbalance, the amplitude of vibration of the equipment may become excessive unless some restraint or limit is placed on it.
There are two well-known techniques for limiting vibration of vibration-mounted equipment under transient conditions. One technique is to provide damping, such as by inherent material damping or light friction damping or, if this is not sufficient, by means of special purpose viscous or friction-type dampers which smooth or flatten the response curve and lower the amplitude of resonant or near resonant vibration of the equipment under transient conditions. Normally, damping in vibration isolators is relatively light, inasmuch as damping increases both force transmission and amplitude transmission from the equipment to the support when the vibration frequency of the equipment is greater than 1.44 times the natural frequency of the equipment, which it normally is. The other technique for preventing excessive vibration under transient conditions is to use stops for physically stopping movement of the equipment in excess of a selected maximum. Special purpose friction or viscous type dampers are rarely used in vibration isolators; the use of such dampers has been confined for the most part to spring-mounted equipment that is vibrated intentionally for a specific purpose, for example, vibrating screens, vibrating conveyors, centrifuges and the like and also for impact machines such as forging hammers, large presses, etc. Light damping and stops have normally been adequate to protect against excessive transient vibration in rotary and reciprocating equipment that is inherently vibratory, but not intentionally so.
The present invention relates to quite a different problem from that of isolating a floor or other structure or equipment mounted on the floor or structure from the normally occurring but undesired vibration of rotating or reciprocating equipment or of intentionally vibrated equipment. Rather, the invention has as an object minimizing the acceleration transmitted to vibration-isolated equipment from unusual forced motion, of a vibratory nature, of the floor or support on which the equipment is mounted, particularly motion caused by a seismic disturbance (earthquake). The problem might best be considered by presenting a typical example.
FIG. 12 of the drawings is a typical curve showing the response spectrum in a given direction (vertical or horizontal) for a particular piece of isolated equipment installed on the upper floor of a building. The natural frequency of a building floor can be calculated or determined in accordance with known techniques, and in the example, it can be read from FIG. 12 to be about 10 cps. The abscissa of the graph in FIG. 12 represents a spectrum of natural vibration periods of the vibration-mounted equipment in question, and the ordinate of the curve represents the input acceleration to the equipment; thus, the curve depicts the magnitudes of accelerations of the vibration-isolated equipment over a spectrum of natural periods of the equipment. The values of acceleration given by the ordinate of the curve presuppose a given input acceleration of the floor, a value that may be determined on the basis of the probability of a seismic disturbance of a certain magnitude, and by considering particular building characteristics. In the example, the input acceleration, as obtained from FIG. 12 (extreme left end of the curve), is 0.25 g.
The curve depicted in FIG. 12 is somewhat analogous to the well-known transmissibility curves that are covered in numerous textbooks on vibration in that input acceleration from the floor to the equipment via the isolators on which the equipment is mounted and the natural frequency of the floor are implicity included in the ordinate and abscissa values on the curve, respectively, although the plots are in terms of acceleration of the equipment and natural period of the equipment. Accordingly, the curve represents the transmissibility of a given input acceleration to the vibration-mounted equipment as a function of a spectrum of natural frequencies of the vibration-isolated equipment under a constant forcing frequency (the frequency of the floor). The curve assumes light damping, and is thus flatter than it would be if damping were not assumed.
The response spectrum illustrated in FIG. 12 itself suggests two possible ways of minimizing the acceleration transmitted to the equipment. The first solution is to provide a natural period for the vibration-isolated equipment that is far to the right (in the curve) of the peak amplitude, preferably a very soft system which, in fact, may reduce acceleration transmitted to the equipment to below the input acceleration of the floor. The selection of very soft springs, however, has the disadvantages of high cost and of requiring a large sway space around the equipment. It would, of course, be possible to accept some amplification of the floor input acceleration, thereby reducing the cost of the isolators and reducing the amplitude of movement of the equipment in response to an input acceleration from the floor, but the spring selection would still be an undesirable compromise between the most economical isolator based on the vibration isolation requirements and the objective of limiting the force and motion transmitted to the equipment in the event of seismic disturbances.
A second solution is to select a stiffer, more economic spring system that meets the requirements of the vibration isolator from the standpoint of isolating the floor from vibration of the equipment and providing stops to restrain movement of the equipment relative to the floor. It is not possible to read from the graph the acceleration transmitted to the equipment when stops are employed, but the acceleration can be determined, and will be greater than the input acceleration from the floor. In general, the stops in such a system would merely limit the amplitude of movement of the equipment, but would not prevent amplification of the floor input acceleration.